In this investigation I will explore the early steps in the mechanism of infection of mammalian cells by the DNA tumor virus simian virus 40 (SV40) and the function of the various capsid proteins in that process as well as their role in virion structure. I will attempt to determine how the virus structural proteins participate in the infection process by the use of deletion and temperature sensitive mutants of SV40 as well as in vitro modification. I will also probe the interactions among the capsid proteins by studying the effects of these deletion and temperature sensitive mutations on virion structure and stability using both protein-chemical and electron-microscopic techniques.